1. Field of the Invention
The present invention relates to implantable cardiac stimulation devices, such as pacemakers and implantable cardioverter defibrillators (ICD) and, in particular, concerns a system and process involving the use of impedance signals to assess electrode locations, particularly during implant.
2. Description of the Related Art
Implantable cardiac stimulation devices, such as pacemakers and ICDs, are commonly used to treat heart abnormalities. Typically, these devices include a control unit and pulse generator positioned within a casing or can and a plurality of leads that extend from the can and into or over one or more of the chambers of the heart. When a heart abnormality, such as an arrhythmia, is detected, the control unit initiates therapy by causing the pulse generator to output electrical pulses or waveforms that are applied to the heart through electrodes on the leads. In order for therapy to be effective, the lead electrodes must be placed in the proper location.
The proper location, however, can vary significantly among patients. More specifically, the evoked response of a particular chamber can vary greatly depending on the position of the lead electrode with respect to the heart chamber. During implantation of the cardiac stimulation device, doctors often look at imagery to ascertain the physical location where the lead electrodes are being implanted. For example, doctors may attempt to implant the lead to position an electrode in the apex of the ventricle. Once the leads are implanted, the device is tested and therapeutic waveforms or pacing pulses are provided to the heart to verify the operation of the device.
Generally, at the implantation stage, a variety of different performance characteristics of the device are adjusted in order to provide a preferred level of operation. For example, the threshold for triggering the device to deliver a therapeutic impulse is adjusted and various delays between sensed events and the delivery of therapy are also adjusted so that the implanted device provides therapy to the patient at desired intervals. While various device parameters are generally adjusted or optimized to achieve a desired level of performance, there typically is not a lot of analysis, other than imaging the location of the implanted lead, to ensure that the lead electrode is positioned to provide a high or optimum level of therapy.
Consequently, while imaging of the lead during implantation provides a general indication that the lead electrode is in the correct location, it is possible that moving the lead electrode to a different location could result in an improved hemodynamic performance of the heart during therapy delivery. Hence, there is a need for a system and method that provides information regarding the hemodynamic performance of the heart during therapy so that an optimal lead electrode location may be determined during implant.